Bui, Tai Duc; (2018) Gas Hydrate Anti-agglomerants: From Molecular-Level Insights to Applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Managing hydrates is one of the most important hurdles in flow-assurance applications. Among various technological alternatives, the use of low-dosage hydrate inhibitors is attracting attention for both economic and environmental reasons. However, it is not well understood how these chemicals function. Molecular simulations were conducted to identify some of the molecular mechanisms that can help prevent growth and agglomeration of hydrates using anti-agglomerants. It has been suggested that the structure of the anti-agglomerants film at the hydrate-oil interface is important in determining performance. Therefore, the structure of anti-agglomerants adsorbed at the interface between methane hydrates and a liquid hydrocarbon was firstly investigated. The anti-agglomerants considered were surface-active compounds with three hydrophobic tails and a complex hydrophilic head that contained both amide and tertiary ammonium cation groups. The tail length, the surface density of the compounds as well as the composition of the liquid hydrocarbon were changed systematically. Some of the surfactants helped maintain a disordered structure at the interface, while others packed tightly, yielding a film that resembled a frozen interface. Comparing qualitatively to experimental data indicates that anti-agglomerants for which a frozen interface was observed exhibited better performance in rocking cell experiments. The results showed that anti-agglomerants films can yield an effective resistance to the agglomeration as well as the growth of gas hydrates, provided their density is sufficiently high. Using non-equilibrium simulations, the anti-agglomerants with short n-butyl tails were found to be able to stabilize hydrate cages and promote growth. However, the penetration of the alkyl tails into the growing hydrate structure helps anti-agglomerants firmly adsorb at the hydrate surface, which is expected to retard hydrate particles agglomeration. These results are crucial for quantifying the hydrate growth mechanism in the presence of anti-agglomerants, which could be exploited for designing new substances with better hydrate-promoting or hydrate-inhibiting characters for different applications.

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